1. Field of the Invention
The present invention is generally related to a liquid storage device, such as a fuel tank and the like. More specifically, the invention is related to a volatile liquid storage device having a thermally insulated emergency relief vent mounted thereon.
2. Description of the Related Art
In the past, above ground storage tanks have been proposed for storing fuel and other volatile liquids. Examples of such systems are set forth in U.S. Pat. No. 5,305,926; U.S. Pat. No. 5,033,637 and U.S. Pat. No. 4,988,020. These patents are incorporated herein by reference. The foregoing storage tanks are primarily intended for use at modern airports having facilities for refueling airplanes. Conventional fuel tanks have been proposed to avoid the need for a below-ground storage tank since these underground tanks require substantial labor and time for installation. Below ground tanks are also less easily inspected to locate leaks. Also, below ground tanks are not movable to different locations.
The above-noted patents have proposed fueling systems containing above-ground tanks. However, above-ground tanks are exposed to environmental conditions, such as the possibility of fire or explosions at the surrounding facility. To address this concern, the '926 patent proposes an above-ground tank having a storage tank received within an outer tank to form an interstitial space therebetween. This interstitial space is filled with a fire retardant insulation material, such as a ceramic fiber blanket, to protect the internal storage tank from external temperature conditions which would otherwise cause the fuel to expand or contract. When a fire occurs proximate the exterior of the tank, the insulation material retards transmission of heat to the storage tank to provide extra time which could be used to extinguish the fire or to escape from the vicinity.
However, the tank configuration of the '926 patent has met with limited success in insulating the interior storage tank from such adverse environmental conditions. The storage tank of the '926 patent, as in most conventional storage tanks, includes a plurality of vents, fill spouts and the like which function as passages affording access to the interior of the storage tank. The vents include emergency vents provided on the upper surface of the tank in communicating with the storage chamber to allow vapor to escape and to afford safe burn off in the event of a catastrophic fire or other emergency. Pressure and vacuum vents are also provided in an upper surface of the outer tank which communicate with the storage tank to open when subjected to predefined pressure variations to allow vapor to escape during normal operating conditions. Similarly, a fill spout and the like may be provided upon the top of the tank. Each of the foregoing vents and fill spouts are formed from a pipe or similar structure which projects radially inward through the outer tank through the insulative material and into the interior storage tank. The vent pipes are generally formed of metal which is highly thermally conductive. In addition, the interior passage of the vent pipes and fill spouts represent a void simply containing air which also functions as a relatively good thermal conductor. Accordingly, during a fire, heat is transferred through the walls of the vent pipes and through the air passage past the insulative layer directly to the shell of the interior storage tank. Such thermal transfer significantly reduces the effectiveness of the insulative layer between the inner and outer storage tanks. Thermal conduction is further enhanced due to the fact that the relief vents are relatively large in diameter, thereby affording passageways having large cross-sections and a pipe having a large circumference to facilitate thermal transfer.
This deficiency of the tank proposed in the '926 patent is of primary importance when using the tank in federally regulated applications. Above-ground fueling facilities in certain applications are regulated under federal guidelines and thus a storage facility must satisfy certain federal safety ratings. By way of example, the National Fire Prevention Association code requires that tanks in some applications, such as aviation fuel storage facilities, have a UL-2085 listing. As is understood in the field, a tank having "UL-2085 listing" has been tested and found to be able to withstand submission to temperatures of 2000.degree. F. for a period of two hours without exceeding a maximum temperature rise of 1000.degree. F. and without exceeding the maximum average temperature rise of 800.degree. F. Achieving a UL-2085 listing is highly desirable as it is one of the higher safety standards.
Tanks such as proposed in the '926 patent have not been able to achieve a UL-2085 listing in a cost effective manner. The tank of the '926 patent utilizes several inches of insulating material between the inner and outer tanks to minimize thermal transfer therebetween. However, as noted above, the thermal insulative effect of the insulation layer is substantially reduced by heat transfer through the relief vents and the like. To compensate for such loss in efficiency, the thickness of the insulation layer has been increased. However, the cost of the fuel tank increases with the thickness of the insulation layer. Accordingly, the tank of the '926 patent has been unable to achieve a "UL-2085 listing" while using a cost effective amount of insulation.
Hence, a need remains within the industry for an improved storage facility which minimizes the transfer of thermal energy between the interior and the exterior of the tank while minimizing the cost of the materials used therein.